As is well known, the use of conventional hypodermic syringes presents a potential health hazard to those employing the syringe as well as to any other health care worker that may handle syringes after use. It is not rare that the person administrating an injection is inadvertently stuck by the pointed end of the cannula after withdrawal of the cannula from the patient or during subsequent handling or disposal of the used syringe. For example, this might occur where a sheath is placed over the cannula of a conventional syringe preparatory to the disposal of the syring. Such occurrences may also happen because of inadvertent mishandling or dropping of the syringe even where good disposal practices are normally used. Such occurrences can of course result in the spread of infectious disease, such as Aquired Immune Deficiency Syndrome (AIDS) or hepatitus.
A number of syringe constructions have been proposed in an attempt to prevent or reduce the number of occurrences of health care personnel being inadvertently stuck by contaminated needles.
In U.S. Pat. No. 4,026,287, a syringe is provided with a needle cannula fixed to the forward end portion of the syringe barrel, which end portion is integral with the barrel but made fragile to break away from the barrel. The end of the plunger upon completion of an injection, is locked to the fragile end portion of the barrel to break the end portion away from the barrel and withdraw the end portion and the needle into the syringe barrel as the plunger rod is retracted. In order to connect the plunger rod with the break-away portion of the barrel, the distal end of the plunger rod is shaped to engage a coupling formed on the inner side of the barrel end portion. Because the plunger must be moved forward when coupling it to the barrel end portion, the residual injection fluid will generally undesireably squirt out the end of the needle. Also, it appears that the breaking of the syringe end portion might be relatively difficult and therefore tend to even contribute to the danger that the person disposing of the syringe will be inadvertently stuck by the needle during efforts to break the syringe end portion. Such a barrel could not practically be made of the commonly used plastic, polypropylene since the breaking away of the end portion of a polypropylene barrel would be very difficult.
A syringe is shown in U.S. Pat. No. 4,692,156 that has a needle secured in the foward end of a syringe barrel by means of a deformable mounting post. The proximal end of the needle is provided with a coupling surface which is engaged by a distally extending end portion of the piston which, after aspiration of the fluid in the barrel, is locked onto the needle. Upon retraction of the piston rod, the needle and the mounting post are retracted into the barrel. With this construction, forward movement of the piston to lock onto the needle will also generally result in fluid being squirted out the tip of the needle. Also, since the mounting post must be sufficiently rigid to hold the needle in its normally exposed position during normal use of the syringe and yet pass through the syringe tip passage when the needle is retracted, retraction of the needle into the barrel may be relatively difficult.
Another type of safety syringe is shown in U.S. Pat. No. 4,666,435. This device utilizes a generally cylindrical shield which receives the syringe barrel and is slidable relative to the syringe so that the shield surrounds the needle after use of the syringe. This construction requires a relatively large shield and complexshaped cooperating features molded on the inside of the barrel and outside of the shield which make the device relatively expensive.